This invention relates to a nondestructive method and apparatus for electromagnetically detecting defects or flaws in metallic objects.
The nondestructive flaw detecting method which utilizes the principle of electromagnetic induction has been widely used for detection of flaws in various kinds of metallic material since the method enables not only quick and accurate detction of flaws but also recognition of the magnitude of the flaws detected. On the other hand, there has been an increasing demand for improvement of the flaw detecting ability of the apparatus, that is, for quicker and more accurate and stable detection of flaws. For improvement of the flaw detecting ability of the apparatus it is required that the probe or sensor should be made as small as possible and arranged as near as possible to the object being tested and in such a manner as to enable quick scanning of the surface of the object.
If the requirements are to be met, however, the stability of the flaw detecting ablity of the apparatus will have to be more or less sacrificed. Therefore, for stability the detecting ability must be kept at a relatively low level, or alternatively a plurality of sensors are provided so that the whole apparatus becomes large-sized with a resulting increase in the cost for installation.
In an effort to overcome the difficulties the present inventor proposed in Japanese unexamined Patent Publication Nos. 53-135378 and 53-135379 both filed, Apr. 30, 1978, non-destructive methods and devices for electromagnetically detecting flaws in objects, in which a sensor in the form of a magnetic core is rotated within a detecting coil. The methods and devices have substantially broadened the area of detection without reducing the sensitivity of detection. However, the clearance or gap (commonly referred to as the lift-off) between the tip end of the rotary core and the surface of the object being scanned is very small and a wider clearance or lift-off is preferred for easier maintenance.
Accordingly, the primary object of the invention is to provide a nondestructive method of electromagnetically detecting flaws in metallic objects, which is simple, has a high sensitivity with a greater lift-off and is capable of covering a large area at one time and detecting flaws extending in all directions.
Another object of the invention is to provide an apparatus suitable for carrying out the above-mentioned method.